Extract generic migration logic into datastores/sql/migrate package

Move database-agnostic migration logic (file discovery, version tracking,
bootstrap detection) into a shared migrate package behind a Dialect
interface, leaving postgres as a thin wrapper.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

datastores/sql/postgres/dialect.go

@@ -0,0 +1,15 @@
+package postgres
+
+import "fmt"
+
+// Dialect implements migrate.Dialect for PostgreSQL.
+type Dialect struct{}
+
+// Placeholder returns PostgreSQL's positional bind parameter ($1, $2, …).
+func (Dialect) Placeholder(n int) string { return fmt.Sprintf("$%d", n) }
+
+// TableExistsQuery returns a query that checks whether a table exists in
+// PostgreSQL using information_schema.
+func (Dialect) TableExistsQuery() string {
+	return "SELECT EXISTS (SELECT FROM information_schema.tables WHERE table_name = $1)"
+}